In the context of commercial aviation, aircraft passengers are often provided with information having instructional and entertainment value. In conventional systems, a relatively limited volume of such information is stored onboard the aircraft and made available to the passengers when the aircraft is in flight. For example, a single in-flight video presentation and several audio presentations are typically stored on tape. This software is loaded into a playback system by an airline employee and playback is initiated at the beginning of the flight. The information is then accessed by the passengers, in part, via headsets located at each seat. As suggested above, this approach offers the passenger an extremely limited program selection. Further, the information is only periodically updated and, when it is, the participation of a software vendor service and aircraft personnel is required.
As an alternative to the use of stored program information, we propose the continuous transmission of information to the aircraft by direct-broadcast satellite. This approach has the advantages of offering a much wider program selection to the passenger on a more frequently updated basis. Such programming would also be more appealing to passengers because it offers continuity with the radio and television fare that passengers are familiar with, and have available, every day. In addition, by eliminating the need for specially developed programs administered by airline personnel and their contracted vendors, it is anticipated that the cost to both airlines and passengers would decrease.
Our proposal does, however, present several problems. First, because the proposed source of transmissions is a geostationary satellite, an antenna provided on the aircraft must be capable of maintaining an antenna beam directed to the satellite as the aircraft traverses its flight path. Although conventional parabolic reflector antennas might be able to satisfy this requirement, their use would impose a heavy drag penalty on the aircraft. As a related problem, the signal-to-noise ratio produced by the antenna may vary undesirably with changes in the aircraft's position.
A second difficulty presented by our proposal relates to the form of transmissions contemplated. Broadly speaking, in communication systems relying upon the transmission of electromagnetic radiation through an unconfined medium, such as the atmosphere, interference between multiple sources and receivers operating over the same portion of the frequency spectrum is a problem. To limit such interference, the government regulates both the frequency and power of such transmissions. With increasing user demand for the available portions of the finite frequency spectrum, efforts have been made to convey greater information over smaller frequency ranges. This is particularly true in the context of satellite communications, where the effective reception area is so great as to effectively preempt all but one operator from using a given portion of the frequency spectrum.
One method developed to use the available portions of the spectrum more efficiently is commonly referred to as "frequency reuse communication". This approach basically involves the use of the same frequencies by two signals conveying independent information and is typically accomplished by transmitting the two signals via electromagnetic fields that are orthogonally polarized. For example, the electric field transmitted by one signal may be aligned perpendicular to the earth's surface (i.e., vertically polarized), while the electric field transmitted by the other signal runs parallel to the earth's surface (i.e., horizontally polarized). Even though the two signals have the same frequency, the information conveyed by each can be distinguished on the basis of its polarity, thus effectively doubling the information-carrying capacity of the spectrum.
As will be appreciated, extraction of information from signals conveyed in this manner becomes significantly more difficult if energy is transferred from one polarization to another. Such "cross-polarizations" may be introduced by irregularities in the transmitter, transmitting medium, or receiver of the system. These irregularities may include, for example, the nonorthogonal propagation of polarized signals at the transmitter, the disruptive presence of elements such as rain in the transmitting medium, and the nonorthogonal alignment of antenna elements at the receiver, each of which will result in the transfer of energy between polarizations. As a result, the signals are no longer easily distinguished by polarity, resulting in transmission interference and less than optimal power transfer.
As will be appreciated, when the efficiency of direct-broadcast satellite is to be increased by frequency reuse techniques, motion of the aircraft may also contribute to cross-polarization of the orthogonal signals. Specifically, although the proposed system will provide a steered antenna beam, aircraft motion may disrupt the effective orthogonality of the antenna elements, resulting in the transfer of energy between the two orthogonal signals. Thus, it would be desirable to provide a system for maintaining the desired polarity of the frequency reuse communications transmitted between a source and receiver undergoing relative motion.
In that regard, various attempts have been made to reduce the effects of cross-polarization occurring in other applications. For example, prior art systems have been developed in which pilot signals are used to detect the cross-polarization. A cancellative form of correction is then employed in which a portion of one of the polarized signals is processed and used to cancel the portion of that signal appearing on the other polarization. Prior art approaches have, however, had the disadvantages of being relatively complicated, requiring cooperative signals to be supplied with transmissions, and possibly requiring separate correction for each of the channels being communicated. As a result, it would be desirable to provide a relatively simple system for simultaneously maintaining the proper polarization of a plurality of frequency reuse communication channels.